The present invention relates to handwriting capture devices and more specifically to an apparatus and method for digitizer sampled point validation.
Pressure sensitive digitizers are devices used to record the position of a data input instrument, such as a pen or stylus, which is pressing on the surface of the digitizer. The digitizer includes two resistive layers which are normally separated, but which come in contact with each other when subjected to pressure from a data input instrument. Separator dots between the layers prevent contact between the layers except at the point of pressure.
Contact resistance between the resistive layers may create significant inaccuracies in point digitization. Contact resistance increases the charge time of any low pass filters coupled to the digitizer. Low pass filters are commonly used to provide protection against electrostatic discharge, noise immunity, and a stable input to an analog to digital converter which takes the position sample. If charge time is longer than the sample period of the digitizer, then the sample is invalid. This type of inaccurate sample point is herein known as a "spike" and the process which produces the inaccurate point is called "spiking".
Commonly employed techniques for overcoming invalid readings due to spiking include oversampling and distance threshold methods. During oversampling, the digitizer takes multiple readings per point and averages them. Disadvantageously, oversampling does not remove the inaccuracies; it merely dilutes them. Furthermore, oversampling reduces the point collection rate.
Using distance threshold methods, the digitizer rejects a point if it is too far from the previous point. However, the distance threshold method cannot filter invalid points that are within reasonable bounds of stylus velocity.
Therefore, it would be desirable to provide an apparatus and method for preventing invalid sample points from being recorded.